I don't follow.
I see % on the right hand vertical scale.
At the 0.1% level the harmonic for a 40Vpk signal (28.28Vac) should be 1/1000 or about 28mV.
The left scale shows values of voltage of 0.0003V (maybe 0.0002828), this is 0.3mV.
That to me seems to be a discrepancy factor of 100times.

Right scale is for THD only. Left scale is amplitude of individual harmonics. They<re not related, I just wanted the THD curve to be "close" to the others in the same graph.

When I look back at it, I notice that the scale on the right should be divided by 100... The amplitude scale on the left is correct as well as the THD curve itself, only the scale is wrong.

I've made some more analysis varying the Rb and vBias and only vBias seems to alter the distortions behaviors. However as already mentionned, the recent simulations i've done demonstrate that a higher value of Rb seems to provide increased stability. In simulation it even permits to lower the miller comp cap without compromising the initial "advised" stability.

As an example, at 10KHZ, with Rb of 22 Ohms, the simulator struggles sometines to plot the oscilloscope trace as rigning appears on the positive side of the wave. Lowering Rb to 10 Ohms makes things worse. Increasing Rb to 56R or 100R erradicates that propblem.
Leaving Rb to 22 Ohms and decreasing Cdom also worsen the problem while increasing Cdom erradicates it as well.

Increasing Rb to 100 Ohms permits decreasing Cdom to 100pF (in place of the 180pF) without compromising stability. I can definitely conclude that there's some kind of relationship between those two. As I'm no engineer on that matter I can only report my observations and draw rough conclusions around it.

My plan is to open the real box and increase Rb to 100 Ohms, increase the bias and test it. One channel at a time. Then when my observations are donw, revalidate the behavior in simulator to eventually try to lower Cdom to 100 pF.

That will bring me exactly at the values advertized by Dr. Self in his base amplifiers.

canonnica, do you mean this Rb? that is interesting, can you keep me updated of your finds I mean if you don't mind, I'm making this layout from Alex MM original design with just a few modifications same circuit just placing them with more space I always like the MKIII from Alex MM layout, but I don't have special equipments nor oscilloscope to check, well that is mister Carlos job I just like layout designs, but is good to know you are checking and find better ways that is good.